Article pubs.acs.org/EF
Cite This: Energy Fuels XXXX, XXX, XXX-XXX
Catalytic Conversion of Chlamydomonas to Hydrocarbons via the Ethanol-Assisted Liquefaction and Hydrotreating Processes Bo Zhang, Lijun Wang,* Rui Li, Quazi Mahzabin Rahman, and Abolghasem Shahbazi Biological Engineering Program, Department of Natural Resources and Environmental Design, North Carolina A&T State University, Greensboro, North Carolina 27411, United States S Supporting Information *
ABSTRACT: Ethanol-assisted liquefaction followed by a hydrotreating process has been applied to the microalgal biomass of Chlamydomonas. The intent of the research was to develop process technology to convert microalgae into drop-in fuels. The operation conditions of the ethanol-assisted liquefaction were optimized using the following variables: reaction temperatures (200−290 °C), ethanol concentration (10−90 vol. %), residence time (0.5−2 h), and the catalyst (SO42−/ZrO2). The application of a higher ethanol concentration and the solid acid enhanced extraction of algal lipids and transesterification. The highest liquid yield of 93.7% for catalytic liquefaction was obtained under the reaction conditions of (290 °C, 90 vol. % ethanol, and 0.5 h). Hydrotreating of the liquid products generated via liquefying microalgae was conducted over a Mo2C/Biochar catalyst at 340 °C and 3.44 MPa hydrogen. The obtained products contained predominantly hydrocarbon molecules falling into the diesel range.
1. INTRODUCTION
hydrodeoxygenation of Chlorella-based FAMEs resulted in a 95% conversion to hydrocarbons.13 Ethanol-assisted liquefaction of lipid-rich (52%) marine alga Nannochloropsis18 and low-lipid (5.8%) freshwater alga Spirulina19 were also studied under subcritical/critical conditions, aiming to produce either fatty acid ethyl esters (FAEEs) or biocrude oil. In this study, a freshwater microalga Chlamydomonas with a 20 wt % lipid content was used as the feedstock for catalytic ethanol-assisted liquefaction to form a renewable oil intermediate, which was hydrotreated over a molybdenum carbide catalyst to yield hydrocarbons. Ethanol can be produced via fermentation of renewable resources such as starch or even algae with a high-sugar content, and is also a good organic solvent. Thus, ethanol-assisted liquefaction of algae may make the combined algae to liquid process (Combined ATL, as shown in Figure 1) more sustainable and economical sound.
Due to growing concerns about declining fossil fuel supplies, environmental issues, and increasing demand for fossil fuels, microalgae-based biofuels have received a large amount of attention during last 10 years. Among all conversion technologies, the hydrothermal liquefaction (HTL) process is considered as one of the most promising approaches for producing microalgaebased biofuels.1 HTL of microalgae can process the whole and wet microalgae, though the biochemical compositions of different species (i.e., protein, lipid, and carbohydrates) have significant effects on the final products.2 During HTL, the microalgal slurry is heated in the continuous plug flow reactor or the batch reactor at the temperature range of 250−374 °C and a pressure of 4−22 MPa for 5−90 min and converted to a water-insoluble biocrude oil.3−5 Biocrude oil produced after the water separation has lower water content and thus higher energy content than that produced directly by the pyrolysis of biomass.6 Biocrude oils consist of hydrocarbons, oxygenated compounds, and chemicals with heteroatoms (like N, S, and P), which might be corefined in an existing oil refinery to produce fuels and chemicals.7 Hydrothermal liquefaction of microalgae has been comprehensively reviewed in the literature.8−12 Organic solvents (such methanol and ethanol) can be applied to assist the thermal treatment of microalgae for multiple purposes, such as extraction of algal lipids, in situ transesterification of algal lipids, and assisting liquefaction. A detailed review on this topic can be found in the literature.13 Recent studies on the application of methanol and ethanol for thermal processing of microalgae are summarized in Table 1. Conversion of algal biomass including Nannochloropsis, Chlorella, and Scenedesmus in the methanol solution to fatty acid methyl esters (FAMEs) has been demonstrated in few studies.13−17 Transesterification of these algal lipids in methanol was done in a temperature range of 60−255 °C without or with a catalyst (such as KOH, NaOH, and H2SO4). A further © XXXX American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Microalgal Strain and Cultivation Conditions. Chlamydomonas sp., which was originally isolated from the local wastewater lagoon,20,21 was grown in 300 L micro-open race way ponds for 15−30 days with commercial fertilizers during the summer of 2016. Microalgae were harvested by a series of operations including settling, siphon, and centrifugation. The collected microalgal biomass was dried at 105 °C until the sample reached the equilibrium moisture content, and then stored at room temperature. The total lipid content of Chlamydomonas sp. was determined as 19.9 ± 4.3% of the cell dry weight.22 2.2. Preparation of Catalysts. A sulfonated solid acid of SO42−/ ZrO2 was prepared for the thermo-treatment process of microalgae. To prepare the sulfated zirconia, concentrated sulfuric acid (H2SO4) was diluted with deionized water to a final concentration of 5%, then loaded onto zirconia oxide supports (ZrO2) by wet impregnation, Received: July 17, 2017 Revised: October 2, 2017 Published: October 2, 2017 A
DOI: 10.1021/acs.energyfuels.7b02080 Energy Fuels XXXX, XXX, XXX−XXX
5 wt % Ni−HY or 5 wt % Pd/C at 300 °C and 30 bar H2 none
none
60 or 100 °C using 5−20 wt % sulfuric acid as a catalyst
245−270 °C and 2−30 min (subcritical conditions) 300 °C for 45 min, no catalyst
methanol
ethanol (about 50−70 vol. %)
ethanol, 50 vol. %
Chlorella (57.3 wt % protein, 17.3 wt % carbohydrates, 2.3 wt % of lipid)
Nannochloropsis salina with a lipid content of 52 wt %
Spirulina (protein 70.3%, lipid 5.8 wt %, and carbohydrate 23.9 wt %)
B
a
ethanol, 10−90 vol. %
FAME: fatty acid methyl esters. bFAEE: fatty acid ethyl esters.
Chlamydomonas sp. with a lipid content of 19.9 wt %
300 or 340 °C for 4 h and an initial H2 pressure of 500 psi
none
70 °C using 5−30 wt % KOH or NaOH as a catalyst
methanol
Chlorella (57.3 wt % protein, 17.3 wt % carbohydrates, 2.3 wt % of lipid)
200−290 °C for 30−120 min with a solid acid catalyst SO4/ZrO2
none
175 °C and after 4 h
methanol, ∼50 vol. %
Chlorella vulgaris
Scenedesmus sp. with a lipid content of 33−35 wt % none
methanol, 90 vol. %
Nannochloropsis sp. with a lipid content of 50% 65 °C for 30 min with 2% KOH as the catalyst
hydrotreating condition
methanol
thermal treatment condition none
organic solvent 255 °C and 30 min, no catalyst
microalgal species
Table 1. Studies on the Application of Methanol and Ethanol for Thermal Processing of Microalgae final products
hydrocarbons
biocrude oil
biodiesel (FAEE)b
biodiesel (FAME) and FAMEbased hydrocarbons
biodiesel (FAME)
biodiesel (FAME)
biodiesel (FAME)
biodiesel (FAME)a
results
catalytic conversion of Chlamydomonas in 90% ethanol resulted in the highest liquid yield of 93% at 290 °C. Hydrotreating of the liquid products produced predominantly hydrocarbon molecules.
liquefaction resulted in the highest bio-oil yield of 59.5% and conversion rate of 94.73%
67% conversion of lipids was achieved at 265 °C and 20 min
maximum biodiesel yield was in the range of 96−98%. Hydrodeoxygenation of this biodiesel gave a more than 95% yield of hydrocarbons
yield of fatty acids was 83% of the total lipid content with oleic and linoleic acids being the main fatty acids
yield is 0.29 g of FAME per g of dry biomass
conversion rate of triacylglycerols reached 100%
conversion ratio of algal lipids to FAME is ∼84%
ref
this study
19
18
13
17
16
15
14
Energy & Fuels Article
DOI: 10.1021/acs.energyfuels.7b02080 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 1. Process flow diagrams for the combined algae to liquid process (Combined ATL). followed by drying in a convection oven at 105 °C for 8 h, and calcinated in a furnace at 600 °C for 4 h. Characterization showed that this catalyst had a BET surface area of 8.3 m2/g, a pore volume of 0.0341 cm3/g, and a pore size of 17.9 nm. The Hammett acidity function (Ho) of the catalyst was determined as ≤3.86. The amount of acidic sites on the catalyst was measured as 0.176 ± 0.04 mmol/g. Biochar supported molybdenum carbide (Mo2C/Biochar) was prepared as follows:23,24 Biochar was the solid residues generated via pyrolysis of loblolly pine sawdust. Biochar was sieved into the particle size range of 0.18−0.25 mm and washed with 0.1 M nitric acid at 100 °C for 12 h. Biochar was then filtered and washed with hot deionized water to remove excess acid until the pH value reached around 7. The treated biochar was dried at 105 °C overnight. Dried biochar was impregnated with an aqueous solution of ammonium molybdate tetrahydrate [(NH4)6Mo7O24·4H2O, 99%, Sigma-Aldrich] to load 15 wt % initial Mo on the biochar. The obtained slurry was dried at a room temperature for 6 h and further dried at 105 °C overnight. The dried material was further carburized in a tubular furnace at an atmospheric pressure purged with argon (99.999% purity) at 100 mL/min. The temperature was raised to 800 °C at 5 °C/min and subsequently held at 800 °C for 2 h to form the biochar supported molybdenum carbide. 2.3. Box-Behnken Experimental Design and Evaluation. The Box-Behnken experimental design uses the variable combinations that are at the midpoints of edges of the process space and at the center.25 In this study, the reaction temperature, the reaction time, and the concentration of the ethanol solution were selected as independent variables. The relative and interactive effects of three variables on the product yields were investigated at three levels. The obtained experimental data was sufficient to fit a quadratic model: n
Y = B0 +
n
was measured with a digital pressure gauge, and then the gaseous products were collected into a 1 L Tedlar bag for gas chromatography (GC) analysis later. Both the final pressure and the composition of the gaseous products were used to calculate the weight of gaseous products. The liquid products and residues were collected into a sample bottle as a mixture. The mixture was centrifuged to separate the liquid products for the residue. The separated residues were washed with 20 mL of ethanol three times to remove oily chemicals and dried at 105 °C for 12−24 h to a constant weight to yield the solid residue. The product yield was defined by the following equations:
yield of reside(%) =
yield of gas(%) =
i F” (i.e., p value) of less than 0.05 was selected, it showed that the reaction temperature and the ethanol concentration were significant factors for the product yields. Besides these two factors, the effects of (the interaction between the temperature and the reaction time, i.e., X1X2) and (the interactions of the temperature/the reaction time and the temperature/the ethanol concentration, i.e., X1X2 and X1X3) were significant to the solid yield and the gas yield, respectively. Three-dimension (3D) response surfaces were performed to elucidate mutual effects of the experimental variables on the liquid yields (Figure S1). These quadratic models were further optimized to find the maximum and minimum values within the studied range of each factor. Since the purpose of this study is to convert microalgae into drop-in fuels, maximizing the liquid yield is preferred. It was found that, when treating microalgae at 290 °C with 90 vol. % ethanol for 0.65 h, the highest liquid yield of 83.6% could be achieved and the formation of solid residue was minimized. Although a higher temperature treatment might result in a higher gas yield, the gas production can be generally limited to a lower extent (240 °C, partial lipids were degraded to short (C3−C5) and medium (C6−C12) chain fatty acids. Several short-chain fatty acid ethyl esters formed. Only the use of the higher ethanol concentration (like 90%) with the solid acid enhanced the complete degradation of microalgal lipids and the formation of medium and long-chain fatty acid esters. The lipid conversion ratio could reach 96% even for a 240 °C treatment. Thus, the use of the sulfonated acid with this simultaneous extraction and transesterification process might convert microalgae to desired liquid products under milder conditions. 3.4. Conversion of Microalgal Lipids. 3.4.1. Direct Treatment. The total lipid content in Chlamydomonas sp. was approximately 19.9 wt % of the cell dry weight. The total lipids include neutral lipids, phospholipids, and lipids with other heteroatoms. In this study, only the content of straightchain fatty acids was quantified. The total fatty acid content of Chlamydomonas sp. that can be quantified was ∼6.5 wt %, and the fatty acid profile was comprised of C4 to C18 fatty acids, in which hexadecanoic acid (C16) was the major component. One purpose of the thermal treatment of microalgae with ethanol−water solutions was to simultaneously extract and transesterify fatty acids to esters. The results of the composition analysis of fatty acids and fatty acid ethyl-esters are shown in Table 5. Treating microalgae with a 10 vol. % ethanol solution could not effectively degrade the microalgal lipids of this species. Even when the treatment temperature rose to 290 °C, the conversion ratio of the lipids was still lower than 5.7%. Applying 50 and 90 vol. % ethanol solutions could effectively increase the conversion ratios of the lipids to 25−31% and 35− 92%, respectively. The highest conversion ratio of 92.4% was achieved experimentally when treating microalgae at 290 °C with 90% ethanol solution for 1 h. ANOVA results (Table 3)
yields were 2−3 times lower than those of direct treatments, representing less than 2.6 wt % of the starting material. Therefore, a higher liquid yield was achieved if using the solid acid during the treatment. The highest liquid yield of 93.7% was obtained when treating microalgae at 290 °C for 1 h with 90 vol. % ethanol, while the lowest liquid yields were still above 70% under the reaction conditions studied. The product yields (Y) of catalytic treatments and three reaction parameters (the reaction temperature X1, the reaction time X2, and ethanol concentration X3) were used to fit response surface quadratic models. Partial ANOVA results are summarized in Table 4. All ANOVA tables and quadratic models are included in the Supporting Information as Table S2. The F values indicated that all quadratic models are significant. When the cutoff “Prob > F” of 90 and >80 vol. % for direct and catalytic treatments, respectively (Table 7). A higher treatment temperature, longer treatment time, and higher ethanol concentration could enhance production of combustible gases such as hydrogen, carbon monoxide (CO), methane (CH4), and ethane. 3.6. Hydrotreating of Liquid Products. 3.6.1. Aging Problem of the Liquid Products. The aging problem associated with liquid products of thermochemical conversion of biomass is well-known. Tables 8 and S5 show the comparison of the chemical profiles of the liquid products before and after 6 months of storage. The chemicals in the liquid products were categorized as hydrocarbons, esters, oxygenated compounds (except esters), nitrogenated compounds, and other heteroatoms compounds containing S, F, Cl, B, P, etc. Although the results were based on the area percentages that cannot be directly correlated to the actual mass, this summary could give us an idea how the chemical profiles might be changed over a long period. Overall, the esters or fatty acid esters were relatively steady, and the species and the total area did not show obvious changes. The patterns of the chemical profile changes were similar for the liquid products from the direct treatment and the catalytic treatment. The most unstable components included mainly nitrogenated compounds and oxygenated compounds, while some hydrocarbons also disappeared during the long-term storage. In order to use the liquid products of thermal treatment of microalgae as transportation fuels or hydrocarbons, a removal of oxygenated compounds and compounds with other heteroatoms is required. For this study, a hydrotreating process was utilized to improve the quality of the liquid products generated from microalgae. 3.6.2. Hydrotreating of the Liquid Products. The liquid product obtained through the catalytic conversion of microalgae in 90 vol. % ethanol at 290 °C for 1 h was further hydrotreated
over the Mo2C/Biochar catalyst in different solvents. The results are summarized in Tables 9 and S6, and GC profiles of all samples are shown in Figure S3. The function of the Mo2C/Biochar catalyst was first studied in the solvents of ethanol and methanol. In the presence of ethanol, hydrotreatment did not show obvious hydrodeoxygenation (HDO) function. Most of ethyl esters were persevered, and the hydrotreated liquid still contained a large amount of oxygenated and nitrogenated compounds. In the presence of methanol, most of the pre-existing ethyl esters and other acids were converted to methyl esters. Meanwhile, the catalyst showed the excellent hydrodenitrogenation (HDN) function, which was indicated by the reduction of the species and the total area percentage (from 30.8% to 1.8−2.8%) of nitrogenated compounds. The major nitrogenated compound in both solvents after hydrotreating was trimethylamine. The existence of methanol and ethanol favored the formation of methyl and ethyl esters, respectively. Because both solvents are oxygen-containing chemicals, the HDO effect of the catalyst was inhibited. Therefore, the liquid products were further diluted in hexane for the hydrotreating tests. The Mo2C/Biochar catalyst exhibited excellent HDO and HDN activities in the presence of hexane. The species of oxygenated and nitrogenated chemicals were reduced at least by half. Meanwhile, the area of hydrocarbons counted for 63.5% of the total area, which was about 7 times as much as that of untreated liquid products. Most of the pre-existing fatty acid ethyl esters were converted to hydrocarbons. For example, the conversion ratio of hexadecanoic acid ethyl ester, which was the primary chemical in the untreated liquid, was 93%. However, there were still many detectable oxygenated, nitrogenated, and other heteroatoms-containing chemicals existing in the treated product. This is probably due to the limitation of the Mo2C catalyst. To achieve the deep cleaning of this microalgal fuel, a hydrogenation catalyst with higher activity and/or a second stage hydrotreatment are needed.
4. CONCLUSIONS This study optimized the operation conditions of ethanol-assisted liquefaction of Chlamydomonas sp., and the obtained liquid G
DOI: 10.1021/acs.energyfuels.7b02080 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels products were hydrotreated over a Mo2C/Biochar to yield hydrocarbons and other products (like biodiesel and ammonia). To maximize the liquid yield of the thermal treatment process, the highest liquid yield of 83.6% was found for the direct conversion under the reaction conditions (290 °C, 90 vol. % ethanol solution, and 0.65 h) through the quadratic model. The reaction temperature and the ethanol concentration were significant factors for the product yields. When applying a solid acid of SO42−/ZrO2 in the treatment process, a higher liquid yield was achieved, and the highest liquid yield of 93.7% was obtained under the reaction conditions (290 °C, 90 vol. % ethanol solution, and 0.5 h). All three reaction conditions and the quadratic effect of the ethanol concentration significantly affected the liquid yield. The chemical composition analysis revealed that a higher ethanol concentration is necessary to degrade the microalgal lipids, and the application of the solid acid could enhance the simultaneous extraction-transesterification process, yielding more fatty acid esters at relatively low temperature of 200 °C. The highest lipid conversion ratio of 96.6% was obtained when treating microalgae at 240 °C for 2 h with a 90 vol. % ethanol solution in the presence of the solid acid. The liquid products of the thermal-treating of microalgae showed obvious aging problems. The most unstable components included nitrogenated compounds and oxygenated compounds, while fatty acid esters were relatively steady during the long-term storage. Hydrotreatment of the liquid products over the Mo2C/Biochar catalyst was conducted in solvents including methanol, ethanol, and hexane. Ethanol was not a suitable dilution solvent and inhibited the function of the catalyst, while the catalyst showed the excellent HDN activity and converted all ethyl esters to methyl esters in the presence of methanol. If diluting the liquid products with hexane, the Mo2C/Biochar catalyst showed good HDO and HDN activities, yielding mostly hydrocarbons. The conversion ratio of hexadecanoic acid ethyl ester, which was a primary chemical in the untreated liquid, was 93%. However, there are still many oxygenated compounds and chemicals with heteroatoms present in the treated liquid. To achieve the deep cleaning of this microalgal fuel, stepwise modification of the existing catalysts and/or a second-stage hydrotreatment are needed.
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products over 6 months. Table S6: Hydrotreating of the liquid products in different solvents. (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Bo Zhang: 0000-0002-4599-7372 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This publication was made possible by Grant Nos. NC.X201338821-21141 and NC.X-294-5-15-130-1 from the U.S. Department of Agriculture, the National Institute of Food and Agriculture (USDA-NIFA). Its contents are solely the responsibility of the authors and do not necessarily represent the official views of NIFA.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b02080. Figure S1: Liquid yield of direct conversion of Chlamydomonas in 3D response surfaces. Figure S2: Liquid yield of catalytic conversion of Chlamydomonas in 3D response surfaces. Figure S3: GC profiles of liquid products obtained via the thermal treatment of microalgae and hydrotreated liquid products. Table S1: Analysis of variance (ANOVA) table [Partial sum of squares - Type III] for response surface quadratic models of the product yields for direct conversion of microalgae. Table S2: ANOVA table [Partial sum of squares - Type III] for response surface quadratic models of the product yields for catalytic conversion of microalgae. Table S3: Composition of liquid products of directly treating microalgae. Table S4: Composition of liquid products of catalytically treating microalgae in the presence of the solid acid. Table S5: The chemical profiles of liquid H
DOI: 10.1021/acs.energyfuels.7b02080 Energy Fuels XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.energyfuels.7b02080 Energy Fuels XXXX, XXX, XXX−XXX
